Aspects of the present invention generally relate to a safety syringe and more particularly to a safety syringe that utilizes a spring loaded outer sheath.
Safety syringes are well known in the art, including from the teachings of WO 2005/089831. Before the syringe is put to use, the plunger disposed inside the barrel is in a starting position, which is a position in which the distal end of the plunger is spaced from the distal front wall of the syringe cylinder or barrel. For drawing up injection liquid, the plunger must first be moved in the distal direction in the syringe cylinder. However, moving the plunger too far distally will cause it to activate the safety mechanism. To limit this plunger movement in the distal direction, limiting arms are mounted at the proximal end of the plunger rod, and these cooperate with the proximal end of the protective cylinder surrounding the syringe cylinder such that the plunger can only be displaced up to just before its distal end position, wherein a small distance to its distal end position remains (FIG. 1). When the limiting arms come to bear at the protective cylinder, they are pivoted no that when an injection is carried out, they no longer prevent the full displacement of the plunger rod into the syringe cylinder, wherein the plunger can be displaced forward up to its distal end or used position at the front end of the syringe cylinder.
In this distal end position of the plunger, the catches at the proximal end of the syringe cylinder engage in recesses in the plunger rod, while they simultaneously release the connection between protective cylinder and the syringe cylinder so that a spring between the protective cylinder and the syringe cylinder displaces these with the plunger rod from the protective cylinder in a proximal direction, as soon as the plunger rod is released after carrying out the injection. Hereby, the needle at the syringe cylinder is drawn into a protective position in the protective cylinder, wherein further notches at the distal end of the syringe cylinder engage in recesses at the proximal end of the protective cylinder, so that the syringe cylinder with the needle can no longer be displaced out of the protective cylinder in the distal direction.
This construction with limiting arms on the one hand and notches at the distal and proximal ends of the syringe cylinder on the other is very complex. It is also deficient in that the design does not permit complete de-aeration of the syringe before drawing up injection liquid without the connection between protective cylinder and syringe cylinder being released (FIG. 1). Rather, the additional operating step of displacing the plunger rod inwards is necessary, which must be carried out contrary to the conventional use of a syringe. If this additional operating step were not carried out, the spring would not release at the end of the injection process.